The moving or advance of the excavator cover end interstice at the rear of a form or casing (liner) structure is a step in the operation of an excavating or tunnel building machine which up to now has not been satisfactorily performed in loose earth or ground.
By "excavator cover end interstice" I mean the gap which arises behind the excavator cover end or tail on advancing the excavator cover and which is bounded by the surrounding ground and the casing, form or liner.
The width of the gap is determined by addition of the thickness of the excavator cover end plates, the excavator cover end seal and the free space for mounting of the casing or form protecting the excavator cover end. This gap which has a width of about 10 cm must be fed or moved with the material injected between the tunnel wall and the liner simultaneously with the advance of the excavator cover to prevent the surrounding earth and ground water from being forced into the space. The disadvantageous consequences could include loosening of the ground around the tunnel so that the support of the form is impaired and the layer of earth above the tunnel drops.
To seal the excavator cover end interstice at the front thereof, joint devices have been built which are connected rigidly with the steel plate of the excavator cover outer surface. The loose earth, possibly with ground water, or the injected concrete should be prevented from being forced in the cover by these joint structures. Up to now, injected concrete has been supplied either through holes in the form or casing segments perpendicular to the tunnel axis or through conduits which are positioned in the cover surface parallel to the tunnel axis. This concrete is pressed into the gap.
However, it can not be reliably guaranteed that the advance pressure for the injected concrete will be greater than the pressure which arises from the weight of the earth and ground water. In fact the space which arises behind the cover end on advancing the cover can not always be filled or compensated by the volume of injected concrete.
Small volume differences which arise, for example by the intermittent operation of the piston injection pump, lead to large pressure differences in the fluidized or fluid injected concrete.
This has the result that both ground water and earth are forced in the cover end interstice whereby nonuniform filling and settling are promoted.
Also a combination of several simultaneously connected and operated ducts or conduits for the injected concrete with a gas reservoir to compensate for the intermittent volume increase of the pump surge does not lead to the desired definite constant pressure in the circular gap of the cover end interstice, Constant pressure is not obtained because the small cross section of the ducts or conduits permits a volume flow of the material fed per unit time which is too small relative to the volume increase which arises by the cover advance of the excavator.